High-efficiency solar cells are required for the inexpensive and competitive generation of power by converting sunlight into electrical power. They should have an efficiency of more than 10 percent, and better still more than 15 percent, and be stable over a period of at least 20 years.
Compound semiconductors of the chalcopyrite type are at present being investigated intensively for solar-cell purposes. High and stable efficiencies of almost 15 percent have already been achieved with polycrystalline thin-film solar cells composed of, for example, copper indium diselenide (CIS). However, yet further cell and process development is necessary for a large-scale industrial manufacture of such solar cells. On the one hand, the development should further simplify the manufacturing process, which requires an expensive process control to achieve high efficiencies of the solar cells produced. On the other hand, toxic compounds, such as hydrogen selenide, cadmium sulfide or cadmium telluride should be eliminated either from the manufacturing process or from the cell structure.
The layer-forming processes disclosed hitherto for producing chalcopyrite thin films can be differentiated essentially on the basis of one-stage and two-stage processes. A one-stage process disclosed, for example, in European Reference EP 678 609 B (Mickelsen et al.) provides a simultaneous vapor deposition of the elements Cu, In or Ga, and Se or S on a heated substrate, the compound semiconductor forming spontaneously on the substrate. In this process, however, the vapor-deposition rate of the elements Cu and In or Cu and Ga and, in particular, their ratio to one another have to be very precisely controlled. This requirement presents considerable problems in the production of large-area solar modules.
In contrast thereto, in the two-stage process disclosed, for example, in European reference EP 195 152 B, the metal layers (for example, Cu and In) are first deposited at room temperature. In a second step, the desired semiconductor compound is formed therefrom by an annealing process in a reactive selenium- or sulfur-containing atmosphere. Although this two-stage process can be extrapolated to large-area coating systems and the quantitative ratios of the individual components can be controlled better, an inadequate adhesion of the semiconductor layer to the metallic back electrode frequently occurs in this process. In addition, a precise temperature profile and an adequate selenium partial pressure must be maintained during the heating of the sample in order to obtain the desired layer properties. A further problem is the highly toxic processing gas, hydrogen selenide, required for the process and, in limited form, also the hydrogen sulfide. In addition, only batchwise manufacture, which considerably limits the quantitative turnover, can be implemented using the prolonged process of several hours' duration.
In a further variant of the two-stage process, the selenium or sulfur component is not incorporated via the gas phase but vapor-deposited directly in elemental form on the metallic layer assembly. Although the formation of the semiconductor proceeds more rapidly than in the abovementioned cases, thin-film solar cells produced therefrom prove unsatisfactory and only have efficiencies of less than 5 percent. The layers exhibit residues of secondary phases and also punctiform peeling from the substrate.
In a further variation of this process, CIS thin films are synthesized using a laser by RTA (rapid thermal annealing) from Cu/In/Se layer assemblies. However, the semiconductor layers obtained in this process were not single-phase, were of excessively small grain size, and were, in addition, n-type.
From a contribution by H. Oumous et al. in Proc. of the 9th EC PVSEC 1989, Freiburg/Breigau, pages 153 to 156, it is known, for example, to heat a stack comprising nine In/Se/Cu elemental layers at 10.degree. C./s to 350.degree. to 500.degree. C. In addition to the chalcopyrite phase, this process also results in foreign phases which cannot be detected by X-ray diffraction analysis. These result in unusable solar cells. The multilayer structure is also expensive to implement in terms of processing and/or impossible to implement on large-area substrates. Excessively small grain sizes and an excessively low layer-thickness uniformity are observed.
A contribution to the 20th IEEE Photovoltaic Specialists Conference, Las Vegas, 26 to 30 Sep. 1988, pages 1482 to 1486, discloses a production process for copper indium diselenide thin layers, in which a sandwich-like stack of thin elemental layers of the starting components on a glass substrate is converted into the compound semiconductor by a laser treatment. This is followed by a thermal treatment in an inert atmosphere.
Solar Cells, volume 30, No. 1/4, May 1991, pages 69 to 77, discloses a process for producing copper indium diselenide thin layers on polycrystalline substrates. The elemental starting compounds are deposited by joint deposition (codeposition) using vacuum vapor deposition and converted into the semiconductor by a rapid heating process.
German Reference DE-A-38 22 073 discloses the production of thin copper indium diselenide layers by thermal treatment of thin elemental layers of the starting compounds.
European Reference EP-A-318 315 (corresponding to U.S. Pat. No. 5,045,409) discloses a production process for chalcopyrite semiconductor layers, in which elemental layer stacks of the starting components are converted into the compound semiconductor in an atmosphere containing hydrogen or hydrogen selenide.
The object of the present invention is therefore to provide an improved process for producing a chalcopyrite semiconductor, which is simple to carry out and is readily controllable and which produces a single-phase and homogeneous semiconductor from which solar cells can be produced which have high efficiencies. In addition, the processing should be safe as a result of avoiding toxic gases.